Fencing around Chicken Coops serves two purposes...
1. To keep your Chickens in and
2. To keep Predators out
Probably the best deterrant is electric fencing, but this can be costly of course. However, left are some Starter Kits for you to take a peep at.
If going down the Chicken Wire method, remember that Foxes teeth are like wire cutters and they could bite through.
However, if the gaps, or holes, of the Chicken mesh are small, the fox is unable to get his jaw through any gap to start knawing.
I would therefore suggest the gaps should be no larger than one inch and get the thickest grade wire possible.
I also suggest a height of at least five feet. Where possible sink the fence at least nine inches into the ground.
Where you cannot sink the fence into the ground, surround the fence with either paving slabs or wide boards. Foxes can scrape and slide under a fence, but they can't tunnel as in going down, along and up.
Note - This usually makes them scale an even higher fence !!!